The present invention relates generally to motors/generators and, more particularly, to high speed switched reluctance machines capable of starting a prime mover as well as generating electrical power for use on aircraft.
The aerospace industry has consistently driven the leading edge of technology with the requirement for lightweight, high efficiency, high reliability equipment. The equipment must be lightweight because each additional pound of weight translates directly into increased fuel bum, and therefore, a higher cost of ownership and shorter range. The need for high efficiency results from the fact that each additional cubic inch required for equipment displaces the amount of revenue-generating cargo and passengers that can be carried on an aircraft. High reliability is important because every minute of delay at the gate increases the cost of ownership, and likewise, increases passenger frustration.
For aircraft electric power generation systems, these pressures have precipitated great advancements in technology, but have also caused problems. Aircraft have typically used synchronous brushless AC generators or permanent magnet generators for electric power generation needs. Unfortunately, both of these types of generators require components which can fail due to the conditions under which they are required to operate (usually mounted directly on the aircraft jet engine).
In addition to an electrical generator, an engine starter is also typically installed on the aircraft engine. This component is used only during starting, which occupies only a very small fraction of each operational cycle of the aircraft. In effect, the starter becomes excess baggage during the remainder of the flight, increasing overall weight, fuel burn, and cost of ownership, and decreasing overall range. This problem has been recognized and efforts have been expended to combine the starter and generator into a single package, thus eliminating the need for an additional piece of equipment used only a fraction of a percent of the time. Unfortunately, using synchronous AC or permanent magnet generators for this purpose, in addition to creating new problems associated with the start function, does not eliminate the inherent problems with these machines as described above.
As an alternative to the use of the synchronous AC or the permanent magnet generator for this combined starter/generator function, a switched reluctance machine can be used. A switched reluctance machine is an inherently low cost machine, having a simple construction which is capable of very high speed operation, thus yielding a more lightweight design. The rotor of the switched reluctance machine is constructed from a simple stack of laminations making it very rugged and low cost without the containment problems associated with rotor windings or permanent magnets. Further, the rotor does not require rotating rectifiers, which contribute to failures, as does the AC synchronous machine.
In order to properly operate a switched reluctance machine, it has been found necessary in the past to determine the rotor position in order to properly commutate the currents flowing in the phase windings of the machine. Resolvers are used, particularly in high speed systems, or sometimes encoders in lower speed systems, to obtain a measure of rotor position. However, resolvers and required associated apparatus (chiefly, a resolver-to-digital converter and an excitation circuit) are expensive and both resolvers and encoders are sources of single point failure.
In order to obviate the need for position sensors, such as resolvers or encoders, sensorless operational techniques have been developed. The most trivial solution to sensorless operation is to control the switched reluctance machine as a stepper motor in the fashion disclosed in Bass, et al. U.S. Pat. No. 4,611,157 and MacMinn U.S. Pat. No. 4,642,543. In an alternative technique, machine inductance or reluctance is detected and utilized to estimate rotor position. Specifically, because the phase inductance of a switched reluctance machine varies as a function of angle from alignment of the stator pole for that phase and a rotor pole, a measurement of instantaneous phase inductance can be utilized to derive an estimate of rotor position. See MacMinn, et al. U.S. Pat. No. 4,772,839, MacMinn, et al. U.S. Pat. No. 4,959,596, Harris "Practical Indirect Position Sensing for a Variable Reluctance Motor," Masters of Science Thesis, MIT, May 1987, Harris, et al. "A Simple Motion Estimator for Variable Reluctance Motors," IEEE Transactions on Industrial Applications, Vol. 26, No. 2, March/April, 1990, and MacMinn, et al. "Application of Sensor Integration Techniques to Switched Reluctance Motor Drives," IEEE Transactions on Industry Applications, Vol. 28, No. 6, November/December, 1992.
In a further technique, phase inductance can be determined using a frequency modulation approach whereby a non-torque producing phase forms part of a frequency modulation encoder. See Ehsani, et al. "Low Cost Sensorless Switched Reluctance Motor Drives for Automotive Applications," Texas A&M Power Electronics Laboratory Report (date unknown), Ehsani, et al. "An Analysis of the Error in Indirect Rotor Position Sensing of Switched Reluctance Motors," IEEE Proceedings IECON '91, Ehsani "A Comparative Analysis of SRM Discrete Shaft Position Sensor Elimination by FM Encoder and Pulsed Impedance Sensing Schemes," Texas A&M Power Electronics Laboratory Report, (date unknown) and Ehsani, et al. "New Modulation Encoding Techniques for Indirect Rotor Position Sensing in Switched Reluctance Motors," IEEE Transactions on Industry Applications, Vol. 30, No. 1, January/February, 1994.
A model-based approach to rotor position estimation has been developed by General Electric Company and is disclosed in Lyons, et al. "Flux/Current Methods for SRM Rotor Position Estimation," Proceedings of IEEE Industry Applications Society Annual Meeting, Vol. 1, 1991, and Lyons, et al. U.S. Pat. No. 5,097,190. In this technique, a multi-phase lumped parameter model of the switched reluctance machine is developed and utilized. However, the model has been developed only for a three-phase machine wound in a north-south-north-south-north-south configuration.
A position estimation subsystem has been developed by the assignee of the instant application and includes a relative angle estimation circuit, an angle combination circuit and an estimator including a Kalman filter. The relative angle estimation logic is responsive to the phase current magnitudes of the switched reluctance machine and develops an angle estimate for each phase. The angle combination logic combines the phase angle estimates to obtain an absolute angle estimate which eliminates ambiguities that would otherwise be present. The estimator utilizes a model of the switched reluctance machine system as well as the absolute angle estimate to form a better estimate of the rotor position and velocity and, if necessary or desirable for other purposes, the rotor acceleration.
The simplest approach is to utilize the estimated rotor position developed by the Kalman filter to directly control commutation. However, estimation of rotor position takes a finite time to calculate and thus, there is a maximum rate at which the calculation can be performed. There is also a processing delay during which the rotor will have turned through some angle depending upon the velocity thereof. Ultimately, the processing cannot be performed fast enough to give the required angular resolution. For example, at 3600 rpm, it takes 46.3 microseconds for the rotor to rotate one mechanical degree. Thus, if the requirement is for one mechanical degree of accuracy, then there is an upper limit set on the maximum rotor speed and/or maximum processing time.
At a minimum, the Kalman filter requires an update rate of twice per electrical cycle so that the rotor velocity can be correctly estimated without aliasing effects. In a 6,4 machine, for example, (i.e., a machine having six stator poles and four rotor poles) this leads to a rotor position estimate update every 45 mechanical degrees of rotation. This amount of rotation, however, is obviously too coarse for use directly by the commutation circuitry.
The object of the present invention is to provide an instantaneous position generation circuit which converts the coarse sampled output of the Kalman filter into a signal having position update intervals which are sufficiently fine to properly control commutation. It is further an object to accomplish the foregoing using circuitry which is simple, reliable and low in cost.
These and other objects and advantages are attained by the provision of instantaneous position generation circuitry including a digitally controlled counter (DCC) which is incremented as a function of estimated rotor velocity of the switched reluctance machine. An output of the DCC is fed back to modify the counter increment to insure that the output of the counter tracks the estimated rotor position accurately.
Preferably, the DCC includes an accumulator having inputs adapted to receive data words each expressed in a relatively large number of bits and further includes an output at which output data words also expressed in a relatively large number of bits are developed. Also preferably, the accumulator is of the high speed type. These features permit the counter to properly control commutation circuitry, even under high speed conditions.
Also in accordance with the preferred embodiment, the accumulator is capable of counting in either of up and down directions and hence the circuit provides compatibility with systems that can operate bidirectionally.
Still further in accordance with the preferred embodiment, the digitally controlled counter has the capability to preload bits at any time during operation so that the counter output can be initialized.
The instantaneous position generation circuitry provides a direct interface between the Kalman filter and commutation logic for a switched reluctance control system. The use of a DCC avoids the need to use voltage controlled oscillators or an array of timers, each of which requires updating before the next commutation instant. In addition, the DCC comprises an easily interfaced replacement for the output of a resolver-to-digital converter.
These and other objects, advantages and novel features of the present invention will become apparent to those skilled in the art from the drawings and following detailed description.